Because we really don't care about your bare opinion. It just raises the question: "Why do you think so?" and you should realize that and answer that question. If you have nothing to add to what has already been said on the subject, then don't say anything. Your comment is devoid of any value, with the possible exception of people that know you and value your opinion even without arguments. Which is questionable in itself. As far as I'm concerned, it deserves -4 to discourage such empty comments.
